Local realism is the idea that objects have de®nite properties
whether or not they are measured, and that measurements of
these properties are not affected by events taking place suf®ciently
far away1. Einstein, Podolsky and Rosen2 used these reasonable
assumptions to conclude that quantum mechanics is incomplete.
Starting in 1965, Bell and others constructed mathematical
inequalities whereby experimental tests could distinguish
between quantum mechanics and local realistic theories1,3±5.
Many experiments1,6±15 have since been done that are consistent
with quantum mechanics and inconsistent with local realism. But
these conclusions remain the subject of considerable interest and
debate, and experiments are still being re®ned to overcome
`loopholes' that might allow a local realistic interpretation. Here
we have measured correlations in the classical properties of
massive entangled particles (9Be+ ions): these correlations violate
a form of Bell's inequality. Our measured value of the appropriate
Bell's `signal' is 2:25 6 0:03, whereas a value of 2 is the maximum
allowed by local realistic theories of nature. In contrast to
previous measurements with massive particles, this violation of
Bell's inequality was obtained by use of a complete set of
measurements. Moreover, the high detection ef®ciency of our
apparatus eliminates the so-called `detection' loophole.